The present invention relates to furnaces, and is directed more particularly to a method and apparatus for increasing the circulating airflow of induced-draft gas-fired furnaces without creating conditions that can result in cold spot corrosion therein.
Furnaces which are of the multi-stage type, i.e., which have two or more stages or firing rates, are ordinarily designed to circulate a fixed quantity of air per unit time through the space to be heated when they are operating at their lowest stage or firing rate. This quantity of air, usually referred to as the circulating airflow of the furnace, is selected not only to maximize the furnace comfort provided to the heated space, but also to optimize the sound level resulting from the operation of the furnace and the durability or useful life of the furnace as a whole. Even the most carefully designed furnaces, however, cannot realize their full potential if they are used to heat spaces that have poorly designed duct systems. Since the inadequacies of duct systems are often not apparent until after they have been installed, and since duct systems can be expensive to modify once they have been installed, heating contractors often try to compensate for the inadequacies of duct designs by increasing the circulating airflow which the furnace establishes when it operates at its lowest stage or firing rate. Heating contractors may take similar measures to deal with duct inadequacies in single stage furnaces.
Operating a furnace at a circulating airflow value which is greater than the circulating airflow value for which it was designed can, however, adversely affect the useful life thereof. This is because increasing the circulating airflow of a furnace has the effect of decreasing the temperature of the walls of the heat exchanger thereof. If the decrease in wall temperature is relatively large, it can allow water to condense on the walls of the heat exchanger, particularly near the outlet end thereof. In furnaces which include secondary or condensing heat exchangers, water can condense on the walls of the coupling box which connects the primary heat exchanger to the secondary heat exchanger even when the increase in circulating airflow is relatively small. This condensed water, in turn, can cause the walls of the primary heat exchanger and/or coupling box to corrode, a condition commonly known as "cold spot" corrosion.
In view of the foregoing, it will be seen that, prior to the present invention, the problem of "cold spot" corrosion has limited the extent to which the circulating airflow of furnaces could be increased in order to offset inadequacies in the designs of their duct systems.